Sets the accel path for this action.
All proxy widgets associated with the action will have this accel path, so that their accelerators are consistent.
Note that accel_path string will be stored in a Quark. Therefore,
if you pass a static string, you can save some memory by interning it first with g_intern_static_string.
| this |
the action object |
| accel_path |
the accelerator path |